In the ever changing world of data communications, there have been advances in the efficiency and complexity of local area networks (LANs). From networks for the simple exchange of data between a few computers, LANs have become reasonably sophisticated multiple drop systems allowing interactive communications between not only computers but a number of other types of devices that are connected to, and operational on, such LANs.
LANs, however, are private, closed systems. This means that the LAN system architecture is optimized around the assumption that only a small, defined number of authorized users will be serviced. Moreover, a single LAN is only meant to service a small geographic area, which is a few miles at most. This is true for both wired and wireless LANs.
The LAN concept has been expanded to cover larger geographic areas and these systems are referred to as metropolitan area networks (MANs). MANs, like LANs, are multiple drop systems but MANs usually have a much larger number of authorized users. MANs also may be wired or wireless.
LANs, both wired and wireless, support the transmission of voice and data signals. The voice signals may be either analog signals or digital signals representative of the analog signals. The data signals also may be analog or digital signals. MANs support voice and data signals like LANs. It would be desirable, however, to have a data communications system as large or larger than a MAN with the attributes of LANs with respect to ease of operations.
LAN and MAN systems usually support random access to the network bus by the authorized user. Depending on the system configuration and operating protocol, these systems attempt to minimize the number of collisions on the network bus by using an arbitration scheme. This device will prioritize use of the network bus among the authorized users. The arbitration function becomes increasingly difficult as the traffic on the network bus increases. Collisions on the bus result in the inefficient use of network bus bandwidth, thereby significantly increasing the latency in transmitting data from a source to its destination. Moreover, as the number of collisions on the bus increase, the ability of authorized users to gain access to the bus decreases.
LAN and MAN systems have used various protocols to prevent collisions on the bus. These include polling, priority requests, carrier-sensing, carrier-sensing with collision-detection, token-passing, and cyclic time-division.
The listed protocols generally solve the collision problem on heavily loaded network buses by allowing only one node at a time to access the bus. As each node relinquished the bus, the protocol would grant another node access to it. Each subscriber requesting bus access would be granted access in due time according to protocol queuing method.
Certain of the listed protocols permitted nonarbitrated access to the bus when the bus was considered lightly loaded. This was because the likelihood of collisions was small. However, the network conditions can change rapidly, so a lightly loaded bus could suddenly become heavily loaded and there would be a large number of collisions on the bus and a substantial loss of bandwidth. Even when it was feasible to use the non-arbitrated access method, there was still only one node at a time given access to the bus.
As systems expanded from LAN type systems to MAN type systems, significant time delays became associated with the system node accessing the network bus. These delays were loop delay times. A loop delay time is the time it takes a transmitted message to be received back by the transmitting node. The maximum loop delay for the system is the loop delay for the node farthest from the head end. Each node had a different loop delay. If any two or more nodes do have the same delay, it was purely by chance.
With the delays, collisions, and other factors that surface as a system approaches the metropolitan area size, it becomes increasingly difficult to find a protocol that serves the needs of high bandwidth and low bandwidth users. In many cases, the needs of these two user groups are in opposition.
For efficient use of the network bus, it is necessary to know the loop delay time for each node and compensate for it in granting a system node access to the network bus. Since loop delay usually is not determined for each system node, to avoid collisions, the arbitration scheme of prior art systems usually waited the maximum loop delay time before granting another node access to the bus.
Community-antenna television (CATV), often referred to simply as cable TV, uses coaxial cable to distribute standard television signals to customers receiving the service. Generally, CATV systems are accessed by greater numbers of users than access either LAN or MAN systems. CATV systems typically include a head end at which signals that are received from the source of programming material are processed for transmission over the system, a trunk system, which is the main artery carrying the processed signals, a distribution system, which is a bridge from the trunk system lines and carries signals to subscriber areas, and subscriber drops, which are fed from taps on the distribution system to feed subscriber TV receivers.
In order to service the large number of subscribers positioned randomly over the very large geographic area covered by a CATV system, the head end has both wireless and wired connections to distribution systems or remote head ends, which connect to yet further distribution systems. These distribution schemes include the use of satellites.
The primary goal of CATV has been to provide high quality TV signals for subscribers. However, today some CATV systems use optical fiber cable to increase the number of channels that can be carried. These systems also have some interactive communications between the subscribers and the programming source, and between subscribers. As a result, CATV systems can carry many more TV channels than ever before, as well as provide other types of communications services on a limited basis.
CATV systems have a spanning tree topology. In principle, this could be adapted to expand the interactive communications capability that now exists in CATV systems. However, CATV systems were not designed for the wide-band communications used by LAN and MAN systems. Moreover, CATV systems are not designed or particularly adaptable to accept data communications formatted for communications over LAN and MAN systems.
There is a need for an interactive communications system that covers a large geographic area and has a larger number of system nodes that marries the attributes of CATV, LAN, and MAN technology but has a bus protocol that permits more than one data packet access to the network bus, compensates for the large loop delays associated with each system node, and can adjust to the changing traffic demands on the network bus, yet appear like a LAN to the user.